Thermally activated heat pumps based on the absorption principle hold great promise for meeting the combined environmental goals of higher energy efficiency (reduced CO.sub.2 emissions) and zero ozone depletion for space conditioning applications. However, the joint achievement of high efficiency, simplicity, and low cost has proved to be elusive.
The use of solid sorbents in single-effect intermittent cycle heat pumps or refrigerators is well known. Solid sorbent use presents the advantages that no sorbent or refrigerant pumps or valves are required (in certain configurations) and the sorbent is reasonably well localized. There are, however, many disadvantages: high latent heat of sorption causes very low coefficient of performance (COP); achieving continuous heat flow requires multiple units connected via complex valving arrangements; the heat release rate tends to be highly uneven, and that coupled with the periodic requirement to change between absorb and desorb results in substantial idle or lightly loaded periods. To compensate for the light load periods, the apparatus must be highly loaded the remainder of the time, and the highly loaded periods determine the heat exchange surface requirements. An additional disadvantage for solid absorbents is their monovariant equilibrium, i.e., pressure is solely a function of temperature, and not of refrigerant (sorbate) content. Thus each solid absorbent (or more accurately each sorbing pair) operates at a unique lift, and if the lift requirement changes, e.g., due to varying ambient temperature, the sorbent cannot adjust. Yet another problem with historical solid sorbent heat pumps was that the characteristic extreme sorbent volume changes (shrinking and swelling) caused the sorbent bed to compact and deactivate. That problem was largely overcome by additions either viscous liquid (LiNO.sub.3) or of various inert conductive media, especially intricate porous structure such as activated carbon. Prior art disclosures of those solutions are found in U.S. Pat. Nos. 2,986,525 and 4,595,774.
For direct-fired space-conditioning applications, the most severe limitation of single-effect solid sorbent intermittent cycles is the low COP. As a result, various multi-effect cycles have been proposed. Unfortunately, they have also increased complexity, by any of several mechanisms: a) sorbate valves and/or throttles; b) sorbent-to-sorbent heat exchange through two heat exchange surfaces; c) complex heat transfer loop valving; d) excessive generator temperature; and e) multiple sorbent beds are interconnected in conjunction with more sorbate than one sorbent bed can hold, which risks liquefying one of the sorbent beds at shutdown or abnormal conditions (all the sorbate migrates to the highest affinity sorbent).
Examples of disclosures of multi-effect solid sorbent heat pumps and their attendant complexities from the above list are: U.S. Pat. No. 5,083,607 (bc); U.S. Pat. No. 5,057,132 (abe); U.S. Pat. No. 2,496,459 (ce); U.S. Pat. No. 5,079,928 (abcde); and U.S. Pat. No. 5,025,635 (abcde).
Rotary sorption heat pumps have been proposed. By arranging a multiplicity of single-effect intermittent cycle sorption heat pumps on a rotating frame, it is possible to achieve continuous heat pumping without either sorbent valves or heat transfer valves. Examples are disclosed in U.S. Pat. Nos. 4,478,057, 4,574,874, and 4,660,629.
The "trisorption" cycle is known in the prior art, although not by that name. It is the solid sorbent analog of a well-known liquid sorbent cycle. The liquid cycle has been referred to as the "Double Evaporation Resorption Cycle", B. A. Phillips, ASHRAE Transactions, Volume 82, Part 1, page 974, 1976. This cycle is characterized by achieving double-effect performance (input heat produces useful refrigerant two times) without need for internal latent heat exchange.
Hans Stymme, "Chemical Heat Pumps", Swedish Council for Building Research, S2:1982, Stockholm, Sweden, 1982 presents an early example of applying solid sorbents in this type of cycle. The essence is that there are three sorbents of differing affinity for the sorbate, and there is a three-step operating cycle, each stage involving a different pair of the three sorbents in both heat and mass exchange, and each step at a different pressure.
Uwe Rockenfeller, et al., in "Complex Compound Chemical Heat Pumps", Proceedings of the 9th Industrial Energy Technology Conference, Sep. 16-18, 1987, Houston, Tex., pp. 158-164 disclose that two trisorption cycle heat pumps can be operated synchronously with phase separation to achieve a nearly continuous heat duty, although with substantial fluctuations, using complex switching of the heat transfer media.
Rockenfeller et al., further disclose actual hardware for accomplishment of the quasi-continuous trisorption cycle heat pump process, at page 64 of "Feasibility of a Complex Compound Heat Pump", Gas Research Institute Report GRI-89/0279, Chicago Ill., December 1991. The schematic flowsheet discloses two reactors containing high affinity sorbent, two medium affinity sorbent reactors, plus a condenser and evaporator (the low affinity reactors), all interconnected in fluid communication via four one-way valves, one expansion valve, and one three-way valve. The six refrigerant valves plus four four-way heat transfer fluid valves are cycled synchronously to accomplish the cycle steps in the required sequence.
M. Lebrun, P. Meyer, and B. Spinner in "Coefficients de Performance de Machines a Froid Monoetagees: 0.8 a 1.6 Selon le Procede de Gestion des Chaleurs de Reaction", Proceedings of the XVIII International Congress of Refrigeration, Aug. 10-17, 1991, Montreal, Canada, p. 567, disclose that the low affinity media can be either a solid or simply condensed phase sorbate, and that the latter generally yields lower Coefficients of Performance.
The prior art multi-effect systems having refrigerant valves tend to be noisy and unreliable. There may be a noisy depressurization each time the valve is repositioned, and a single leak anywhere in the system can cause the entire system to fail and release a large quantity of refrigerant. Also the unsteady power level in each step of prior art trisorption cycles causes the heat transfer fluid temperature to vary during the step, and is wasteful of fan and/or pump power.
Conversely the prior art systems without valves and with steady power levels (e.g., the rotary systems) are not multi-effect. Co-pending U.S. patent application Ser. No. 905,284 "Rotary Trisorption Heat Pump" filed by Donald C. Erickson on Jun. 26, 1992 discloses one means of avoiding all refrigerant valves and discontinuous operation while retaining double effect performance with solid sorption heat pumps. A plurality of valve-less hermetically individual trisorption modules are mounted in a rotating frame, and frame rotation interposes the respective sorption zones appropriately and successively in heat transfer fluid conduits conveying hot, moderate, or cold temperature fluids.
The limitation of the above-cited application is the large rotating mass, albeit at a very slow rotation. The large rotating mass requires large bearings and large heat transfer fluid seals. Also, when conditioned air is one of the heat transfer fluids, there may be a need for a double wall boundary to preclude releases of NH.sub.3 at irritant levels. Use of liquid heat transfer fluid may be difficult with the large seals. With gaseous heat transfer fluids, it is more difficult and expensive to convert from winter heat pump mode to summer cooling mode, and also from trisorption (multi-effect) mode to single effect.
What is needed, and included among the objects of this invention, are apparatus and corresponding process for at least one of heat pumping and refrigeration which achieve the high COP multi-effect performance characteristic of the trisorption cycle in simple and reliable equipment which has no refrigerant pumps or valves, no sorbent-to-sorbent heat exchange, and which has continuous and nearly constant delivery of heating and/or cooling. There should be only a minimum number of control mechanisms or moving parts (e.g., valves) in the heat transfer loops, preferably only one or two for fluid distribution, plus when desired one for heating/cooling changeover and one for single effect/multi-effect changeover. There should be no large moving assemblies--the only moving parts being the three or fewer control mechanisms plus any required motive mechanisms (pumps or fans) for the heat transfer fluids. The movable heat transfer fluid distributor mechanism preferably has a reliable and simple seal face. The apparatus preferably should have no critical height or orientation requirement, and not rely on gravity for maintaining critical vapor-liquid interfaces or liquid drains. It should be modular, whereby a refrigerant leak from one module will not cause the entire apparatus to cease function.